Such filter elements filter fluids, liquids or solutions of any sort, for instance in the form of true suspensions, dispersions, emulsions or colloidal solutions. The filter elements have been proven to be generally useful on account of their compact structure, combined with their large filter surface. To guarantee that the fluid stream flows only through the proper filter and not elsewhere, the connecting fold, incorporating the two ends of the filter mat facing one another adjacent to the sealing on the end casings in the form of webs or fillets at the ends, forms one of the critical points which are to be sealed off and leak-proof.
Known filter elements have a zigzag folded or pleated filter mat composed of various filter materials. The mat is arranged around a solid support pipe mounted in the interior of the filter element and provided with passages, and is protected against damage from the outside by a fine-mesh wire meshing following the shape of the folds of the filter mat. The connection of the filter mat with the protective wire meshing is costly in construction, and therefore, is expensive. Also, the filter mat itself is not completely protected from damage. The wire meshing projecting out from the assembly can lead to injuries to operators during the handing required, for instance, during filter exchange.
The known wired meshing can absorb only low-level forces. The filter element can eventually bulge out and become useless, as soon as the direction of flow, for instance during a reverse flow step, is reversed to come from the inside to the outside. Since the wire meshing is directly in contact with the outside layer of the filter mat, a resistant filter material is selected for this purpose, in order to avoid damage to the filter mat. Such a resistant material will also have good passage and filtering properties, such as for example sensitive filter materials made of polyester, fiberglass or paper fleece, which is not available. Since the wire meshing is also of considerable mesh spacing, metal splinters and other polluting particles can damage the sensitive filter material and might negatively influence the filtering performance to a great extend.
The Italian company ITALDIM S.P.A. headquartered in Bergamo has already suggested, in a filter element for this type of use, that the zigzagging filter mat can be surrounded with a cylindrical expandable metal grid on the exterior. The filter can be composed of sensitive, but even more effective filter materials, solely by use of the wide-mesh expandable metal grid in the known arrangement. However, problems arise of limited pressure stability. Also, the filter mat is not protected from damage. With use of the wide-mesh grid in this case too, there is danger of injury to the operator, especially when metal splinters or the like have been caught and held in the mesh of the fabric.
In this known solution, the ends of the cylindrical expanded metal grid are flush with one another and are point-welded or flat-welded together. This arrangement is costly and requires considerable outlay in the manufacture. The reverse side surface of the sensitive filter mat can be damaged by heat application or by formation of sparks. Furthermore, the construction of this type of connection requires a relatively extensive surface area on the exterior peripheral side of the expanded metal grid. Thus, this area is no longer available as a filtration surface.
In European Patent Specification Nos. 0 139 822 A2, 0 440 352 A2, 0 001 407 B1 and 0 062 867 B1, and from German Patent No. 31 28 546 C2, other types of filter elements are disclosed. Longitudinal seam clamps either surround or even engage directly in the two ends of the filter mat which face one another, in order to be subsequently heat-welded together with the longitudinal seam clamp. In this manner, very solid and leak-free connections of the free ends of the filter mat are attained. However, the manufacturing method is costly. Besides, the selection of material for the composition/construction of the filter mat is limited. Only certain materials, such as plastics, which can be subjected successfully to heat processing, can be used.
In other filter elements, disclosed in German publication No. 2 417 551 and British Patent No. 750 396, metallic, shell-shaped longitudinal seam clamps are known receive the ends of the curving filter material forming the filter mat. A liquid, adhesive material or a casting resin having two components is used to connect the members. The adhesive material can be processed at room temperature, so that the use of a destructive level of heat is avoided.
In another filter element, according to U.S. Pat. No. 4,512,892, following production of the adhesive bed by means of the shell-shaped longitudinal seam clamps, the clamps can be removed. Two installation aids can be provided in the form of members which serve as additional side boundaries for the adhesive bed. These two installation aids are nonporous and are installed around the final curve in the design, where they serve in the form of clamps in the end area of the filter mat where the ends overlap. The adhesive agent does not advance further in the filtering area of the filter mat because of capillary effect, having a negative influence on the filtering capacity or performance.
The aforementioned other filter elements at least sometimes are provided with a shell-shaped longitudinal seam clamp. The filter mat is not protected from damage from the outside, for example, particularly not during transport and construction. During operational use, the mat is not sufficiently pressure-stable. The operator is not secure from injury during exchange of the filter element, insofar as sharp-edged small parts, such as metal splinters, have been set and secured in the filter mat. Since the filter mat is the only filter, this filter mat can be rapidly polluted with the resulting small rejected parts, which considerably lowers the filtering performance capacity. Additional, the life of the known filter element is shortened considerably.